1. Field of the Invention
The present invention relates to a device for changing the rotational position of a rotating body in response to a command electrical signal. The driving device of the present invention is used, for example, for controlling the rotational position of a movable member of a device.
2. Description of Prior Art
In general, an electromagnetic device is used for actuating a movable member of, for example, a valve.
An example of prior art electromagnetic driving device for a valve will be explained with reference to FIG. 1, attached to the present specification. The device of FIG. 1 includes a coil 101, a plunger 102, a spring 103, a valve 104 incorporated with the plunger 102, a seat 105 for the valve, an input port 106 and an output port 107.
When the coil 101 is energized, the plunger 102 is attracted to the leftward direction, and, hence, the valve 104 separates from the seat 105 to place the valve in the released state. When the coil 101 is de-energized, the plunger 102 returns to the rightward direction because of the force of the spring 103, so that the valve 104 comes into close contact with the seat 105, placing the valve in the closed state.
In the device of FIG. 1, the amount of current supplied to the coil 101 to realize the leftward direction motion of the plunger 102 must be great enough to overcome the resilient force of the spring 103. At the same time, the force of the spring 103 must be great enough to realize quick rightward direction motion of the plunger 102 for quick valve closing. However, the leftward direction motion of the plunger 102 for releasing the valve becomes slower as the force of the spring 103 increases. Thus, the speeding-up of the releasing motion of the valve counteracts the speeding-up of the closing motion of the valve, and, hence, the response speed of the valve cannot be increased above a predetermined speed.
Another prior art device, disclosed in Japanese Unexamined patent publication (Kokai) No. 55-33041, published Mar. 8, 1980 consists of an on-off action valve constructed by using two coils, i.e., without using a spring for the restoring motion. This on-off action valve is not satisfactory, however, because two coils are required and because the size of the device is increased because of the increased length of the plunger.